This invention relates to a ventilatory instrument for measuring Peak Expiratory Flow of a human subject.
The best known form of portable mechanically operable instrument for measuring Peak Expiratory Flow is the Wright Peak Flow Meter. Essentially it comprises a pivoted vane within a drum. Rotation of the vane inside the drum is opposed by air drag and a resisting spring. A radial inlet leads directly into the drum. A forced expiration through the radial inlet by a human subject under examination causes the vane to rotate, allowing the breath to escape increasingly from the drum through a peripheral slot. The vane comes to rest at a position which depends on the peak flow which has been attained. The vane is held at this position by a brake which can be released by pressing a button adjacent to the radial inlet. A pointer indicates the movement of the vane relative to a graduated scale. The scale reading indicated by the pointer when the vane comes to rest is a measure of the Peak Expiratory Flow of the human subject under examination.
Another form of portable mechanically operable instrument for measuring Peak Expiratory Flow is disclosed in GB-A-1160669 and GB-A-1344836. It comprises a cylindrical tube containing a piston. There is a spring which resists movement of the piston away from an inlet which is formed at one end of the tube. There is a longitudinal exhaust slot in the cylindrical tube. A forced expiration through the inlet by the human subject under examination impinges on the piston and propels the piston along the tube. The resistance of the spring and air drag on the piston oppose the expiration which is vented by the progressively increasing area of the exhaust slot. The piston is held by a releasable brake at a position corresponding to the peak flow attained and a pointer, which is carried by the piston and which projects through the slot, indicates a reading on a calibrated scale which runs alongside the slot, the indicated scale reading being a measure of the Peak Expiratory Flow of the human subject under examination. GB-A-1463814 discloses an arrangement whereby the releasable brake can be dispensed with, and which comprises, instead of the pointer that is carried by the piston, a separate indicator member which is mounted in the slot for movement therealong and which is moved away from the inlet by engagement by the piston but which separates from the piston upon movement of the latter towards the inlet, the arrangement being such that the piston travels to the point at which the venting of the body balances the expired air input and then subsequently returns under spring action while the indicator member remains to mark the point of maximum travel relative to the scale.
WO84/02642 discloses a ventilatory instrument for measuring peak expiratory flow of a human subject, comprising a hollow casing within which a vane is journaled at one end so as to extend across the interior of the casing, the casing forming slot means and having an inlet by which a forced expiration by the human subject is admitted into its interior and is directed onto a major surface of the vane to cause a radially extending edge of the vane to traverse the slot means which comprises an arcuate array of spaced slots, whereby to increase the amount of the slots between the vane and the inlet through which breath can escape from within the casing, the clearance between the radially outer portion of the vane and an adjacent portion of the casing within which the slots are formed, being substantially the same throughout the travel of that outer vane portion, and there being a separate indicator member comprising a pointer which is journaled substantially coaxially with the vane and which is adapted to be pushed by the vane as the latter is moved away from the inlet but which separates from the vane upon movement of the latter towards the inlet, the arrangement being such that the vane travels to the point at which the venting of the body balances the expired air input and then subsequently returns due to the action of resilient means while the pointer remains to mark the point of maximum travel. The pointer is inside the casing. Hence one has to be able to see through the front face of the casing on which a scale is depicted, in order to read the recorded peak expiratory flow that is indicated by the pointer on the scale. In practice, the taking of that reading is difficult because the interior of the casing, and especially of the front face, mists up due to condensation from the warm moist breath blown into the casing by the human subject as he uses the instrument.
U.S. Pat. No. 2,892,348 discloses a flowmeter for measuring and indicating continuous flow of liquids, slumes or gases in a conduit or pipe. It uses a rigid vane which is mounted at one end by an integral curved leaf spring within a fluid-tight casing. As the flow increases, the vane moves in the casing against the loading of the spring and the space between its tip and the adjacent wall of the casing increases. There is a magnetic coupling between the vane and an indicator pointer which is outside the casing. U.S. Pat. No. 3,955,415 discloses a flowmeter for the measurement of small rates of flow of liquids or gases, comprising a hermetically sealed housing having a transparent wall on which a scale is provided and a thin flexible resilient vane attached by one of its ends to the casing so that it is deflected to sweep the scale by flow through the casing, the position of the vane relative to the scale being a measure of the flow that deflected the vane. As with the flowmeter disclosed in U.S. Pat. No. 2,892,348, the space between the tip of the vane and the casing increases as the flow increases.